This invention relates to thermoformed articles such as trays and a method for improving their preparation and handling properties.
There has been much work developing polyester compositions which have improved crystallization rates. For example, U.S. Pat. No. 4,380,621 discloses a fast crystallizing polyester in which at least some of the end groups of the polyester have the formula --Y.sup.- M.sup.+, where M.sup.+ is an alkaline metal ion, preferably sodium. The compositions can be prepared by polymerizing the polyester in the presence of an alkaline metal salt or corresponding base, such as NaOH. The composition can be compounded with various additives, including an oxidized polyethylene wax containing carboxylic acid groups. The composition is useful preparing molding powder for use in preparing injection molded articles.
U.S. Pat. No. 4,801,640, discloses a polyester film with improved dimensional stability, abrasion resistance, drawability, and slip properties. The film contains hardened organic particles such as melamine/formaldehyde resin and a nucleating agent selected from various alkali metal salts, including salts of ester waxes.
U.S. Pat. No. 4,365,036 discloses fast crystallizing polyalkyleneterephthalate resins containing a crystallization promoter which is a complex of sodium iodide and a polymeric compound, together with fillers or additives including paraffin. The product is useful as a molded composition.
U.S. Pat. No. 4,448,913 discloses polyethylene terephthalate molding compositions characterized by rapid crystallization, provided by combining PET and a filler or reinforcing agent with a crystallization rate promoter comprising sodium benzoate and a poly-alpha-olefin such as low density polyethylene. Examples of the composition include small amounts of wax.
U.S. Pat. No. 3,960,807 discloses thermoformed articles such as trays from thermoplastic, crystallizable polyester, a polymeric crack stopping agent such as poly-4-methylpentene-1, and a nucleating agent such as talc.
U.S. Pat. No. 4,463,121 discloses walled thermoformed articles of partially crystallized polyethylene terephthalate containing a minor component of a polyolefin which exhibit improved impact resistance. Traditional nucleating agent (such as talc) are not required.
Japanese publication 57/025352 discloses a polyester resin composition with improved mold release property, comprising glass fiber, a metal chelate such as sodium acetylacetone chelate, montan wax, and PET resin. The composition has excellent moldability, mold release property, and luster.
Japanese publication 59/109551 discloses a dimensionally stable polyester resin composition prepared by adding montan wax ester to glass fiber reinforced PET. Good mold release properties are imparted.
Japanese publication 57/096,039 discloses a polybutylene terephthalate resin composition including ammonium polyphosphate and paraffin. The resin composition has good mechanical properties and external appearance.
Japanese publication 55/154,130 discloses a slidable polyester film containing micropowdered silica coated with polyolefin wax. The blend is extruded into a sheet which is then stretched.
Japanese publication 56/109,245 discloses a polyethylene terephthalate resin composition containing e.g. polyethylene wax. A crystal nucleating agent such as sodium benzoate or stearate, etc., may be present. The composition exhibits a glossy appearance even when molded in a low temperature metallic mold.
Japanese publication 53/143,651 discloses rotary articles obtained by blending polybutylene terephthalate resin with a wax which is solid at ordinary temperatures.
Plastic trays suitable for oven use have become increasingly popular in recent years. A commonly used plastic for making such trays is crystalline polyethylene terephthalate (CPET), since it provides good high temperature properties and can withstand cooking at e.g. 200.degree. C. or above for 0.5 hours or longer. Often CPET compositions include crystallization promoters such as low density polyethylene in levels of up to several percent, to provide rapid crystallization for ease of processing. However, the use of such blends of polymers have certain disadvantages. In particular it has become apparent that polymeric blends are generally less readily recyclable than are compositions consisting of substantially a single polymer. In view of the current interest in providing polymeric articles which are readily recyclable, it is desirable to provide a composition for trays and other thermoformed compositions comprising polyethylene terephthalate as substantially the only polymeric component.
In addition to such recycling concerns, PET compositions containing other polymers can present processing difficulties. For example, PET pellets almost always need to be dried at high temperature before melt processing. Pellets of polyethylene, however, cannot withstand high temperatures without melting. Thus formed articles which contain a significant amount of polyethylene cannot be readily prepared from a "single bag" mixture of pellets of PET and the polyethylene. Furthermore, it has been found that polyethylene in such blends can lead to deposits on casting rolls and vacuum forming equipment. In order to avoid these difficulties, the present invention provides articles made from CPET which retain processability and rapid crystallization without addition of polyethylene.